The present invention relates generally to sensing and detecting apparatus, and more specifically to compact, self-contained solid-state apparatus for resolving electromagnetic spectra such as separating light into constituent colors and detecting selected groups of colors, typically used in scanners, electronic cameras, detectors, and the like.
Spectrally resolving an electromagnetic signal, for example light, into its constituent wavelengths, for example colors, is well known. Typical apparatus for doing so include prisms, diffraction gratings, thin films, etc., and many applications have been made of the ability to spectrally resolve such signals into their constituent parts. Electronic imaging, filtering, and object recognition are several of the more common applications. Electronic imaging applications are of primary concern herein, including those that operate primarily in the visible light region of the electromagnetic spectrum, and those that operate primarily outside that region. (For purposes of the present application "spectral" shall be used to mean both visible and nonvisible regions of the electromagnetic spectrum.) Electronic imaging applications operating primarily in the visible light region include, for example, video cameras, facsimile machines, electronic copiers, etc. Electronic imaging applications operating primarily outside the visible light region include infrared (IR) or ultra violet (UV) detectors, spectrum analyzers, etc. The aim of these electronic imaging applications in general is to convert an electromagnetic signal (hereafter referred to as a "source image") into a machine manipulable data representation thereof.
Apparatus for producing machine manipulable data representations of a color source image include means for performing at least two functions, filtering or resolving the source image spectrum, and detecting selected portions of the filtered or resolved source image. Heretofore, these functions have been performed by separate means. For example, U.S. Pat. No. 4,786,964 to Plummer et al discloses an electronic imaging apparatus including separate filtering means and detector means. Multicolor striped or mosaic optical filters filter all but selected spectral components of the source image. Typically, 3 different color filters are employed to distinguish the primary colors. For an additive process, red, green, and blue are commonly used. For a subtractive process, yellow, magenta, and cyan are preferred. Although not specified, these filters are typically gelatin filters (such as dye inside a polyimide coating) as known in the art. These filters are placed over a plurality of charge coupled devices (CCDs) which detect the intensity of the light transmitted by each filter.
The general assembly and operation of the apparatus according to Plummer et al. is representative of the state of the art of color electronic imaging. The device of Plummer et al. happens to be a camera, although other references such as U.S. Pat. No. 4,734,760 to Futaki and U.S. Pat. No. 4,580,889 to Hiranuma et al. disclose other applications of this general operation. In general, however, filtering means (as opposed to resolving means) are used to separate the spectral components of the source image. The difference between the two, as further discussed below, is that filtering means reduce the available image intensity as a function of the number of components to be detected, whereas resolving means allow utilization of the maximum image intensity available, regardless of the number of components to be detected.
One variation on the above involves use of multiple light sources of different color to illuminate an object such as a color document. Light will be absorbed by the object in regions of similar color to the source, and reflected otherwise to produce a source image. Sensors such as the above-mentioned CCDs, photodiodes, or the like may then be used to detect the extent of reflection for each light source color, and by additive or subtractive processes the color composition of the object may be approximated.
Another variation on the above general assembly and operation is disclosed in U.S. Pat. No. 4,709,114 to Vincent. A color source image is caused to be incident upon a stack of dichroic plates which are reflective to selected colors and transmissive to all others. Sensors are positioned such that selected reflected color components of the source image, reflected by one plate of the stack, are individually detected. Alignment of the sensors is crucial in this arrangement in order to distinguish the sensing of individual colors.
Yet another variation of the above-described general embodiment is disclosed in U.S. Pat. No. 4,822,998 to Yokota et al. The filtering means disclosed in Yokota et al. comprises a silicon dioxide body formed to have areas of step-wise increasing thickness to define discrete filtering elements which, taken as a whole, form an interference filter. The greater the thickness of the filtering elements, the longer the transmission wavelength. The sensing means disclosed in Yokota et al. are arrays of photodiodes mounted or formed on the surface of a substrate. These photodiodes may be provided with different sensitivities to operate in conjunction with the filtering elements for sensing selected color components. The interference filter is mounted in either touching or spaced apart relationship to the photodiode arrays such that transmission by each element is caused to be incident upon a photodiode.
Each of the devices of the prior art have shortcomings and disadvantages which have been addressed by the present invention. One problem common to all the above-mentioned apparatus is that most of the light intensity of a given wavelength is not delivered to the sensor intended to sense that wavelength; on the contrary, most of the light intensity of a given wavelength is wasted. Transmission filters such as gelatin films filter light by transmitting certain colors of light and absorbing all others. Gelatin film transmission efficiency is at beast on the order of 50% in the range of colors they are designed to transmit. Furthermore, in order to filter a color source image into a number of components, say N discrete components (N is commonly referred to as the number of bins that source is divided into), there will be at least N filters. Some portion of the source image must fall on each of the filters (i.e., into each bin). If evenly distributed, there will be at best 1/N times the intensity of the source image falling on each filter. Once filtered, there will be at best 50% of this amount falling on the sensing means. The dichroic filters and interference filters have a much higher transmission efficiency than gelatin filters, however, they must also divide the source image N times (into N bits), where N is the number of components to be detected, thus reducing available image intensity by a factor of N.
One aspect of the present invention is the realization that, by utilizing a much greater fraction of the available source image intensity than provided by the prior art, device performance could be enhanced. For example, in a color scanner, scan speed is limited by the rate at which the sensor devices can build up a sufficient accumulation of photogenerated electron-hole pairs (alternatively, the time it takes for a sufficient number of photons to strike the sensor surface). One way to increase this rate is to allow more photons to strike the sensor devices in a given period of time. Thus, for a given lamp intensity, a scanner whose resolving means filters little if any light can scan at a faster rate than one whose resolving means filters portions of the source image intensity. Similarly, for a given scan speed, the lamp intensity may be reduced if more efficient use can be made of the source image intensity; lower power lamps enable smaller power supplies and hence cost reductions. This increase in performance applies not only to scanning devices, but to electronic cameras, sensing devices, and a host of other electronic imaging applications.
Another problem not addressed by the prior art is the presently unfilled need for a full spectrum resolving and sensing apparatus. That is, it is desired to be able to divide the spectrum into a relatively large number of detectable components. The ability to divide the spectrum in this manner facilitates many advantageous uses of spectral information, including mathematical or physical manipulation of the components for various purposes such as conversion of the spectral data into the standard Commission Internationale de l'Eclairage (or CIE) tristimulus values, detection and utilization of subperceptual coded data, compensation for the effects of a colored light source in color original scanning (which allows greater freedom in the selection of light source), measuring the spectral content of ambient light, etc. A practical device capable of spectrally resolving a polychromatic source image into an arbitrary number of elemental components has heretofore been unavailable. The gelatin and dichroic filter arrangements are practical for only a very small number of filters due to the filter size, alignment of filters with sensors, and other limitations. For these filters, additive or subtractive processes are employed to construct the broad color spectrum. The same holds true for multicolored light source apparatus. The technique employing a staircase of deposited-film interference filters allow a large number of components to be separated, but resolution of these devices is limited by the process used to form the lands or steps of the interference filters (limiting the number of lands), diffraction effects in the interference filter, alignment of the interference filter with the sensing elements, etc.
It will be noted that the above prior art relates to filtering. Filtering may be generally defined for the purposes of the present invention as selective removal of portions of a spectrum so as to acquire other selected portions of that spectrum. By spreading the source image into a continuous spectrum, rather than filtering it, a great number of components (i.e., wavelengths) of that spectrum may be sensed. Resolving for the purposes of the present invention may thus be defined as decomposing a source image such that its components (i.e., wavelengths) may be presented spatially separated from one another into a continuous spectrum. Thus, another aspect of the present invention is the provision of an apparatus able to resolve a source image, and able to detect virtually an arbitrary number of elemental components of the source image.
The alignment problem discussed above deserves further mention. In several of the prior art devices discussed above, the filtering elements and the sensing elements are formed separately, then joined. Gelation filters are generally on the order of an inch or less in size. They are often positioned over a great many number of detectors, so that alignment of these filters over the prior detectors, although important, is not critical. Dichroic filters are of a similar scale to gelatin filters, and apparatus incorporating dichroic filters require varying degrees of precision of alignment, but their most common application like that disclosed in U.S. Pat. No. 4,709,144 to Vincent require only a rough alignment to assure that the sensing means receive reflected light from the filters. However, apparatus which use interference filters such as U.S. Pat. No. 4,822,998 to Yokota et al. require more critical alignment of the filter over the detectors. The scale of such devices is small--on the order of 5-10 mm square. Each land or step of the interference filter must be located over at least one preselected detector. Thus, it is another aspect of the present invention to alleviate the need to align the filtering or resolving elements and the sensing elements, or to form either or both in such a way that they are self-aligning.